The objective of this research is to show that prezygotic selection is an important selection mechanism to maintain various genetic polymorphisms in nature. Two genetic systems were chosen. 1) The ABO blood group system in man (for families residing in northern Japan). A large set of family data was carefully collected, and evidences have been accumulated for strong selections still operating in this system. It is now possible to estimate parameters associated with various components of fitness and to construct the overall fitness of each ABO type. Emphasis will be placed on the role of prezygotic selection. 2) The male recombination system (Mr) in Drosophila melanogaster. As an experimental organism, Drosophila melanogaster will be used in parallel to the ABO study. The frequencies of Mr in natural, and in laboratory populations are very high (20-50%), indicating that some genetic mechanisms are operating in favor of Mr. Searches will be made for specific selection mechanisms, especially for prezygotic selection. This system is a complex system consisting of several elements. Some of the elements behave in peculiar ways similar to those systems which show paramutation, transposition, or extra chromosomal nature. The genetic nature of these phenomena will be also analysed.